Paragraph 18 states: 'Due to the risk of delayed cracking, a period of at least 16 hours is generally required before the final inspection is made of as-welded fabrications. The minimum time may be reduced for thin materials below 500N/mm2 yield strength or increased for materials of thickness greater than 50mm or of yield strength over 500 N/mm2 ... Welds that have been heat-treated to reduce hydrogen content or which have been stress relieved, need no additional time interval following the heat treatment before final inspection is made'

Paragraph 5.5.4 'Hold Times before final NDT' states: 'If there is a risk of delayed cracking, a period may be needed before the final inspection is made of as-welded fabrications. Recommended minimum hold times are given in Annex A.'

Table A Recommended minimum hold times

Table A Recommended minimum hold times

Weld size (mm

Heat input (kJ/mm)

Hold time (hours)

a or s ≤ 6

All

Cooling period only

6 < a or s ≤ 12

≤ 3

8

> 3

16

a or s > 12

≤ 3

16

> 3

40

Size applies to the nominal throat thickness (a) of a fillet weld, the nominal weld depth (s) of a partial penetration butt welld, or the nominal material thickness (s) of a full penetration weld.

If two fillet welds are separated an unfused root face of less than 10mm then the governing weld size (a) shall be taken as the sum of their individual weld sizes.

Heat input to be calculated in accordance with clause 19 of BS EN 1011-1.

The time between weld completion and commencement of NDT shall be stated in the NDT report. In the case of 'cooling period only' this will last until the weld is cool enough for NDT to commence.

Paragraph 6.11 states: '...Acceptance criteria for ASTM A 514, A517, and A709 Grade 100 and 100W steels shall be based on NDT performed not less than 48 hours after completion of the welds...'. (These are all 690MPa yield grades, and for other grades, inspection immediately after completed welds have cooled to ambient temperature is permitted.)

Offshore standard DNV-OS-C401, October 2010

Chapter 2 Section 3 paragraph B105 states that 'Final inspection and NDT of structural steel welds shall not be carried out before 48 hours after completion, except where PWHT is required. The time delay may upon agreement be reduced for NV36 grades or lower and for NV420 grades or lower for plate thicknesses less than 40mm, if consistent low failure rate of delayed cracking has been documented for the materials and welding consumables in question'. (NV36 and NV420 have minimum yield stresses of 355N/mm2 and 420N/mm2 respectively.)

Further information

Some recommendations for the delay time for a range of conditions are given in the abstract below. See the full paper

'Evaluation of necessary delay before inspection for hydrogen cracks'

Introduction

In the welding of ferritic steels, the most common form of fabrication cracking is that caused by hydrogen embrittlement. It is well known that there can be some delay between the completion of welding and the formation of hydrogen cracks in ferritic steels. If, therefore, inspection is carried out too soon after welding, these cracks may not be detected, with potentially catastrophic consequences. On the other hand, excessive delays after welding prior to inspection can have serious financial implications due to, for example, hold ups in production.

Procedure

In this test programme, multipass butt welds were made in a variety of steels, at near threshold hydrogen cracking conditions, and the development of hydrogen cracking was monitored using ultrasonic examination. For the majority of the work, welds were stop ended bead in groove, in 50mm thick plate. Using this basic geometry, effects of parent material and welding consumable, heat input, hydrogen level and restraint were explored. Some trials were also carried out at higher heat input using larger submerged arc butt welded panels.

Results and discussion

Data generated using a 690MPa yield low alloy steel confirmed that maximum delay between completion of welding and initiation of cracking is experienced at near threshold conditions. In this steel, delays of up to 21 hours were recorded before first detectable cracking, with new cracks continuing to appear for up to 64 hours, and crack growth continuing for up to 140 hours after completion of welding.

In 350Mpa and 450Mpa yield C-Mn steels, the delay between completion of welding and crack initiation in groove welds was generally less than the time taken to set up monitoring equipment, although one new crack was detected after 4.7 hours. Crack growth was seen to continue for up to 6 hours.

Hydrogen level, restraint and weld size had no measurable effect on delay times, but there was an apparently linear increase in delay times in the C-Mn steels as heat input increased from around 0.7-5kJ/mm.

Conclusions

Based on the above results, recommendations have been made for minimum delays which should be imposed between completion of welding and inspection for hydrogen cracking in butt welds of up to 50mm thickness, as summarised in the table below: